In such pumps, it has been shown that, despite predetermined relative measuring of the control arrangements which define the pre-injection fuel quantity during operation, a desired pre-injection fuel quantity which remains constant throughout the whole engine speed range is not achieved. It is apparent that pre-delivery and post-delivery effects in the pump are responsible for an undesirable increase in line pressure on the high-pressure side of the pump, and hence in the pre-injected fuel quantity, as operational speed increases.
The present invention seeks to eliminate this disadvantage and to keep the pre-injection fuel quantity at a substantially constant level for the whole range of speed or operating states.